The present invention relates to protective relays protecting circuitry subjected to a battery connected to the circuit with reverse polarity and against damage caused by over voltages and xe2x80x9cload dump.xe2x80x9d xe2x80x9cLoad dumpxe2x80x9d occurs upon sudden disconnection of a depleted battery from a motor vehicle electrical system. The invention is particularly advantageous in automotive applications.
Utilization of solid state smart power switches is increasing in automotive applications. The vast majority of these components are built around a power MOSFET transistor and auxiliary circuits. When properly employed, these power MOSFETS are extremely reliable, supporting over-current, over-temperature, etc.
However, a major weakness of the power MOSFET surfaces when the device is operated under reverse battery condition. In this situation, the parasitic diode formed in the MOSFET construction conducts, turning on the loads connected to them with potentially harmful results such as short circuit of the supply lines, blown fuses or damage to the wiring harness. In some cases, there is no damage to the majority of the circuit, but the switch destroys itself due to the fact that the reverse conductive current capacity is lower than the forward conductive current capacity.
A popular solution is to put a diode in series with the portion of the circuit to be protected, often an electronic module within the automobile, in order to block the current caused by the reverse voltage. Unfortunately, this solution is only practical for Low-power modules. For modules requiring higher power, the diode would need to be extremely large and expensive.
One solution for high-power modules is to mount a diode in anti-parallel with each MOSFET switch in order to shunt excess reverse current. Another solution is to put one diode in series with each individual switch so as to block current in a reverse voltage situation because of the reverse bias of the diode. Unfortunately, in this case the forward bias voltage drop of the diode, during normal operation, will result in heat dissipation and will reduce the voltage available to the load. And, whereas the first solution does not interfere with the normal operation of the circuit, it can cause overheat problems in cases of prolonged exposure to the reverse battery condition. In addition to their limited effectiveness, these additional protective components also increase size and cost of electronic modules. Further, both solutions do not add any improvement to the operation of the circuits during normal operation (i.e., proper battery connection). These protective components are only beneficial in the limited circumstance of reverse battery connection.